SMT-BD2
4 – I
2
t PROTECTION
4.1
–
C
URRENT LIMITATION IN
F
USING MODE
When the amplifier RMS current (I
2
t) reaches 85 % of the
Rated current
, the Idyn signal output is activated and
the I
2
t error display is blinking on the amplifier front panel. If the RMS current (I
2
t) has not dropped below 85 % of
the
Rated current
within 1 second, the I
2
t fault is released and the amplifier is disabled (otherwise, the Idyn signal
and the blinking I
2
t error display are both cancelled).
When the amplifier RMS current (I
2
t) reaches the
Rated current
value, the I
2
t protection limits the amplifier
current at this value.
The amplifier current limitation diagram in an extreme case (motor overload or locked shaft) is shown below.
t0
t1
t2
t3
Rated current
Maximum current
Amplifier current
t1 = Idyn signal activation
t2 = current limitation
t3 = I
2
t fault
1 second
Time
The maximum current duration before the release of the Idyn signal depends on the value of the
Rated current
and
Maximum current
parameters. This value is calculated as follows:
T dyn (second) = t1 - t0 = 3.3 x [
Rated current (%)
/
Maximum current (%)
]
2
The maximum current duration before the limitation at the rated current also depends on the value of the
Rated
current
and
Maximum current
parameters. This value is calculated as follows:
T max (second) = t2 - t0 = 4 x [
Rated current (%)
/
Maximum current (%)
]
2
NOTE 1
The above formulas are valid as long as the
Maximum current
/
Rated current
ratio is higher than 3/2. When
the
Maximum current
/
Rated current
ratio is close to 1, the calculated values of Tdyn and Tmax are quite
below the real values. For example when
Maximum current
/
Rated current
= 1.2, the measured Tdyn = 3.4
seconds and the measured Tmax = 4.4 seconds. When the
Maximum current
/
Rated current
ratio is equal to
1, the I
2
t protection is no more disabling the amplifier but the current is limited at the
Rated current
value.
NOTE 2
The amplifier I
2
t signal can be displayed on the digitizing oscilloscope by selecting the "
I
2
t
" signal in the
“
Channel
” menu. The I
2
t signal threshold values according to the I
2
t protection mode described above are
calculated in the following manner :
Idyn signal activation threshold (%) = [
Rated current (%)
]
2
/ 70
Current limitation threshold (%) = [
Rated current (%)
]
2
/ 50
The corresponding amplifier RMS current value can be calculated according to following formula :
Amplifier RMS current (%) = [
I
2
t signal value (%)
x 50]
1/2
!
I
n
Fusing mode
, the amplifier
Rated current
value must be adjusted lower or equal to the
Maximum authorized rated current
of the amplifier (
see Chapter 2, section 1
).
51
Chapter 8 - Appendix